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Sample Deadlock

Translucent areas depict waiting for something; incomplete lock statements have dashed border. Note that it doesn't matter in which order the top two acquisitions are made.

Can a cryptic entanglement of your mutex locks lead to a deadlock? It sure can. Deadlocking is the second thing your parents tell you about mutexes: if one thread acquires A, then acquires B before releasing A, and the other does the same in the reverse order, the threads may potentially deadlock. And deadlock they will if the first two acquisitions are picked from separate threads. Here's the code of two threads, and sidenote depicts the problematic execution:

But what about Reader/writer locks also known as "shared/exclusive" locks? Let's recap what these are first. Sometimes, to achieve greater efficiency, a mutex implementation supports two flavors of locking: Reader and Writer (otherwise known as "shared" and "exclusive"). If several threads only want to read from a shared variable, there's no need for each of them to wait for others. That's where you'd use a ReaderLock operation on a mutex guarding the variable. If thread wants to write, it invokes WriterLock which means "do not run any readers of writers while I'm holding the lock". Here's a wiki entry for reference, and here's standard Java API.

Seemingly OK Reader Lock Execution

We no longer have a "must-before" relation between B locks in two threads, so they don't deadlock. This looks OK, but it actually is not!

So imagine that both threads X and Y happen to use one of the locks as Reader lock? It seemingly should prevent deadlocking: if, say, B is a reader lock, then the execution specified above will make progress: B.ReaderLock() in thread X will not block waiting for thread Y to release it... right? Here's the code for clarity:

Turns out, reader locks can deadlock. You just need to make reader lock wait for another reader lock's release; how?

Many mutual exclusion implementations make acquiring threads "form a line" of some sort to ensure fairness: no thread should wait forever for a lock. Then a threads that tries to acquire a lock--either shared or exclusive--waits until all threads that called L.WrLock() earlier exit their critical sections. Fairness is especially important when you have reader and writer locks: if you'd allow any reader lock to proceed while there is another reader holding the lock, your writers could "starve" waiting for quiescence among readers, which may never happen on a highly contended lock.

So, to make a reader lock wait on another reader lock, we need a writer lock between them.

Deadlocking Reader Lock Execution

Here's how three threads can interleave such that you have a deadlock between reader mutex locks. The "blocked by" relationship between these reader locks transitively hops over a writer lock in some other thread Z.

Assume, that in the execution described earlier, before Thread X attempts to acquire the reader lock B, thread Z chips in, invokes B.WrLock(), and only then X calls B.RdLock(). The second X's Y.RdLock() starts to wait for the Z to acquire and then release B because of fairness concerns discussed above. Z's B.WrLock() waits for Y to release B.RdLock(). Y waits for X to release A. No thread makes progress, and here's the deadlock. Here's sample code of all three threads:

Note that you will have at least one writer lock for B somewhere (because if all acquisitions are Reader Locks, there's no point in having the lock at all.) Therefore, the only way to prevent this kind of deadlock is to not distinguish reader and writer locks when reasoning about progress guarantees.

This kind of deadlock needs at least three threads in order to bite you, but don't dismiss it outright! If the Internet taught us that a million monkeys in front of typewriters will not eventually recreate all the body of Shakespeare's work. they would at least trigger all possible race conditions in our typewriters no matter how contrived the corresponding executions seem.

"Forget these silly runtime errors you had in Ruby: C++ is statically-typed!" Levap, a seasoned engineer, was enlightening the new hire. "You no longer have to wait while the sample run or unit tests finish to learn that your variables have the wrong type, nor you have to worry about this happening in production".

So, Paul, the younger engineer, stopped worrying. He went back to implementing a subclass that, unlike its parent, would only perform a low-level blocking write of data stored in a buffer object instead of more complex writing semantics found in the parent:

The new Write function would assert that the second parameter was false, and would just call a regular system's write, familiar to any C programmer. The only exception was that it fetched the target from the SimpleWriter object's private variables, and it wrote data chunk-by-chunk until finished. Hence, it didn't have to return the number of bytes written, and threw a fatal error on a severe error.

The yonger engineer soon found out that creating NeverBlockingWriter in place of SimpleWriter only required modifying method, and should involve no data copying. In a dynamic language, he would do just that by replacing the SimpleWrite method of the object with a nonblocking-only version, but in C++, Paul had to instantiate a whole new class and copy the information from the parent one to it. An alternative could be to force the superclass to be friends with its own child, which sounded more suitable for a dumb scene in a sitcom. Oh, if he only had the powers of Ruby...

Paul first merely called the parent's SimpleWrite(buffer, block), but then decided that calling SystemWrite directly was clearer and faster, so he fixed the code shortly afterwards:

This code looked right, but didn't work. What was worse, Paul planned to get the code working on the last hour before Christmas, and he couldn't, which created a sense of incompleteness and incompetence for the whole holiday.

What happened was that Paul updated the name of function to call, but forgot to update its arguments. Normally, C++ would barf a compile error in an instant. In this case, however, the trap was perfect: C++ silently, without a single warning, cast the pointer to Buffer to pointer to void, and boolean true to one. The program expected the other endpoint to reply to the data written, so the lack of the complete write created a very weird bug. Apparently, these functions were not different enough for C++ to show a compile error.

The fixed version immediately demonstrated that everything else was OK:

"Statically typed, my ass," were Paul's next words. Nobody heard him, since he stayed later to fight this bug.

Paul saved the code, and finally left the office. On his train, he shed a tear for OCaml that distinguished between integer and boolean, and promised that next time he saw a weird bug, he would check if he fell into another loophole in C++'s "static" typing.

"Lol, just pay more attention next time, smartass," the seasoned engineer advised.

When you develop something in Ruby on Rails, you will most likely encounter strange errors here and there. These errors may look like language or framework bugs, but they are not. Most likely, they are programming bugs you were not aware of before you encountered them.

The reasons for that are twofold. First, Ruby is a dynamically-typed language, and this alone introduces subtleties you should know about. Second, the web framework values convention over documentation. It means that methods and functions should just work as expected, without making the user read the whole documentation page.

And when something just doesn't work, you'll have troubles finding the cause in the API docs. You'll have to google the exception message, and hope to find the cause.

This post is another contribution to this Exception-Driven Development, and lists some errors a Ruby/Rails programmer may make. Some of the errors were found via web search (I tried to provide a proper attribution where possible), and some of them were solved by my own. I'll try to keep this list up-to-date, and add more errors here as I write more Rails programs.

Generic Ruby Issues

Calling setter methods

You are setting a value of an ActiveRecord field from inside a method of your model, but it does nothing? You are doing it wrong. Perhaps, you write something like this:

But the value is reset after this method. What's happening? The field = ... line actually creates a new local variable, rather than calling the object's field=(value) method ActiveRecord has created for you. Just use self to distinguish:

Note that you don't have to do the same when you read from the field—unless you have created a local variable with the same name! This is why the second line of the method doesn't have to contain self in this case.

Sometimes this strikes you from a different angle. For instance, if you have a quality attribute in your model, and you want to set conversion quality with RMagick, you have to write:

because you can't easily access the outside's variable quality from inside the block, as it's routed to the RMagick object instead.

Other gotchas

You might have specified :false instead of false when you pass a hash attributes to Ruby. I devoted a separate post to this.

you should clear your cookies. Perhaps, you have deployed a different site under the same hostname. If this alone does not help, check if you have set up constant DOMAIN or SESSION_DOMAIN in your config (I had them in config/environments/production.rb), adjust them, and restart your server.

Strange remote_ip error

This error:

undefined method `remote_ip' for nil:NilClass

means that you accidentally named one of your actions "request". And you can't name your actions "request", as there's some clash when the engine wants to get a current request, but calls your controller method instead. Rename your action.

Spawn plugin and Rails 3

That means that it's too old for your current version of Rails. Upgrade it—to the latest commit it has (this is not on the master, but on its edge branch), or use it as a gem. If you use Rails3, here's the line for your Gemfile:

gem 'spawn', :git => 'git://github.com/tra/spawn', :branch => 'edge'

Commit 0e1ab99b worked for me, the corresponding gem version is 1.1). Source.

Unicode problems

UTF-8 regular expressions do not work by default? Add #coding: utf-8 as the first line of your ruby file. If it's an ERB or HAML template, add this at the beginning anyway.

If you want to make your application completely Unicode, from the database to the tips of your views, you should do all of the following.

If you miss one of the steps, you may encounter problems. For instance, if you do everything except for the mysql2 gem, you may see these errors for your views:

Showing app/views/layouts/application.html.erb where line #48 raised:
incompatible character encodings: ASCII-8BIT and UTF-8

If you do not set up the character set for the database, migrations may create new tables with latin1 encoding, and this may result in ?????????? instead of non-ascii lines in the data user enters to your DB.

Models, Databases, and Validations

Ever-failing presence validation

Do NOT add empty? method to your objects unless you really mean it! This can strike you from you back when you don't expect.

Assume you want to validate that a "belongs_to" association presents at save. You might use validates_presence_of for this. However, as specified in the documentation, it calls Rails' Object.blank? to check presence. It, in turn, calls empty? method (documentation) if it exists.

In my forum engine, I used empty? method to convey if the post's body is empty. Therefore, you couldn't answer to posts with empty bodies, because validation thought that such posts are nonexistent.

Instead of using a complex validation, I suggest renaming the empty? method. When the author of the next model forgets—or won't even happen to know—about this bug, he/she will still try to use validates_presence_of. It's better to make it work, following the principle of least surprise.

Model name clashes

You can't name your models like Ruby classes, i.e. Thread (conflicts with internal class for threading) or Post (conflicts with HTTP library).

It's interesting, that the default scaffolding automatically names your model "Threads" when you instruct it to create a "Thread" model to avoid conflict. However, you still need to specify the proper class name in associations:

Check the scaffolded controller for other declarative methods that may also need adjustment.

Using several databases in the application

If you want to specify several databases for your app (on a per-model basis), you can use a built-in ActiveRecord capabilities. Just add a section in your config/database.yml with a distinguished name (i.e. second_db instead of production), and add this to your model:

Troubles with making a form with validation but without database

Rails3 has a built-in solution for creating a form that use ActiveRecord-style validations, but are not backed up by a DB table. Indeed, not reusing this functionality would be a waste. But just including ActiveModel::Validations makes your form yeild errors like this:

For ActiveRecord attributes, these methods are automatically generated. However, for our attributes that are not backed by DB columns, we should create on our own.

Uniqueness validation column name error

I created a model, added an `validates_uniqueness_on`—uniqueness validation—and tried to create a valid record. However, I was getting the NoMethodError: undefined method `text?' for nil:NilClass error no matter how hard I tried:

I checked the code, and realized that my uniqueness validation referred to an association name, instead of the database column name:

It seems that you must specify the exact database column name in the uniqueness validations since they are closely integrated with the database.

Connecting to a local database via TCP sockets

If you want to avoid using file sockets for connection to a local database, it's not enough to just remove the socket line. Rails will replace it with the default socket configuration if you use localhost as your host. To connect via a network socket, you have to change "localhost" to something else by adding an entry to /etc/hosts, for instance.

Housekeeping

Accessing methods from console

Contrary to the popular belief, there is a way to easily access helpers and other useful application methods from console. Here is a couple of examples:

Get list of Rake/Capistrano tasks

When you use rake tasks, or a similar program (such as Capistrano), it's easy to forget what actions it provides. Googling for the list doesn't help either. Where can you find them?

They're right there:

These commands print the list of all tasks. With cap -e task:name, you can get a more extended help.

Redo specific migration

$ rake db:migrate:redo VERSION=20111122143422

Rake can also go several steps back/forward in migration application; you now know how to see the docs (rake -T).

Using Ruby 1.9 on Ubuntu

Ruby 1.9's default virtual machine is superior to that of 1.8. Besides, version 1.9 has

If you are using Ubuntu, you have "alternatives" mechanism for setting ruby version. But. please, do not forget to update alternatives both for ruby, and for gem command to 1.9! Otherwise your gems (such as passenger) may end up in an improper place, and won't run successfully.

MySQL gem and Mac

If you install mysql onto a MacOS with MySQL 5.5, you may encounter problems that show up as a strange exception:

rake aborted!
Constant MysqlCompat::MysqlRes from mysql_compat/mysql_res.rb not found
Constant MysqlRes from mysql_res.rb not found
Tasks: TOP => db:migrate
(See full trace by running task with --trace)

It turns out that the problem here is a wrong mysql gem version. Uninstall the existing gem, and install 2.7 version, fixing some errors afterwards with a symlink:

Using Ruby 1.9 on Ubuntu

Ruby 1.9's default virtual machine is superior to that of 1.8. Besides, version 1.9 supports OS-level threading, and some multithreading bugs fixed.

If you are using Ubuntu, you have "alternatives" mechanism for setting ruby version. But. please, do not forget to update alternatives both for ruby, and for gem command to 1.9! Otherwise your gems (such as passenger) may end up in an improper place, and won't run successfully.

As a result, my jQuery didn't work. It sometimes worked, but its AJAX events did not fire although the server reported that queries had been made. The mistake was that I should had not downloaded and placed the jQuery js file into the assets folder. I should only had added the "require" lines into the application.js.

Talking to Web APIs

Specifying POST and GET PARAMS at the same time

If you are to POST some HTTP data to a URL that has GET parameters, documentation used to elide this case. The correct way to do this is to create URI object, and supply it as a whole, not just uri.path.

Converting data structures to POST form data

It's strange that Rails doesn't have such a method (I couldn't find it in version 2.13). When you invoke API of a web service, you sometimes need to convert arrays, hashes and other structures to a result that looks like a HTML form that Rails itself uses. For instance, this structure:

should be "flattened" to a "form field name"-"form value" set like this:

The "solutions" found in the internet seem to have been written by programmers who are not aware of recursion. Here's the correct one I wrote:

Russian Support

Here's a gem that adds Russian support to your Rails messages in default form validation and helpers. Russian morphology differs from that of English. In this plugin, the issues with date and proper clause of plural nouns are solved, though the difference in nouns on form create/update nouns is not.

Programming languages have strict and mostly very compressed syntax. The origins of that are twofold.

First, actively used programming languages have a considerable history, and were started decades ago. Back then, screen space was limited, and, before that, there was little storage available for source code. Second, programmers have technically-oriented minds and like to endow abstract symbols with complex abstractions instead of using words to describe them. That's why, I guess, many programming languages heavily depend on correct usage of individual characters. Even as few characters as one may make a difference in program behavior as well as in whether program compiles.

Such caveats in different languages include:

C++ templates. You can't write map<int,vector<int>> when you define a mapping from integers to arrays of integers, because C++ parsers thinks that >> as a bit-shift operator in an improper place. A correct program differs by a single space between the < signs.

Makefile tabs. A rule body in the Unix Makefiles should be indented with a Tab. Spaces do not work. Being a software with more than 30 years of history, makehad it fixed only a year ago.

CSS delimiters. When you define a cascading style sheet, amd want to define the same block of style attributes for a certain class inside a certain tag, you write the selector as tag .class. It's just a space away from tag.class that defines the styles only for tag elements of class class.

C-style equality in conditional statements. If you're a seasoned professional, you should have already forgotten about this. In many languages, including C, Ruby, Python, Javascript, C# and others, if (a = 1) is always true, since it's an assignment of 1 to a, followed by checking the a's value for truthfulness. The correct version of that has one extra equality sign: if (a == 1). More confusion is created by languages, where the first version is the legitimate way to compare values, such as Pascal.

Imagine how surprised I was when I realized that I made a variation of mistake #4 in my Ruby code today! Here's what it was about.

Ruby hash niceness

Named parameter is a way to specify function arguments at call by name (rather than by order, as in the standard function call notation).

Here you may find how the Named Parameter is implemented in various programming languages.

To emulate a Named Parameter Idiom, Ruby uses hashes and some syntax sugar. The last parameter of a function may be a hash, which maps from parameter names to values. Syntax sugar allows a programmer to write

This sugar is not unique to Ruby; Perl also supports it.

instead of

The :name notation specifies a symbolic constant, which effectively is an immutable string that is defined by only one ancillary character.

Such hashes and symbols seem as a very useful feature. It allows you to emulate DSL-s; here's an example of Ruby on Rails web framework routing configuration:

Until one day, after an hour of debugging you find yourself having written something like this:

See what's wrong here? Indeed, here's how the code of the function called might look like:

So, options[:attribute_check] should evaluate to false boolean value, but... :false is a totally different thing; it's an immutable string of five characters that evaluates to true instead! Just one colon that lurked into the code, and it made it behaving very wrong way.

Just like in a C-style typo, some expressions that are evaluated as true in boolean context look like those that are evaluated as false, and you should be careful with the borderline.

New named attribute definition style in Ruby

New named attribute passing style was not designed to address this problem. However, the abundance of colons in such a code makes it look worrying in case there is a mistake like the above:

You see that the mistake is easy to spot, because the conjunction between the name and the parameter value is so ugly, that it immediately draws attention. However, if you actually need to specify symbol as a value, then you'll have to look ugly with this style.

Moreover, you can't erase the space between the parameter name and value, because for this code:

Ruby parser will think that it's false method in an attribute_check object, as :: is a scope resolution operator, just like in C++. Space matters again, as in typo #1 desccribed above.

People say that this style resembles that of C# or JSON. So, maybe, it is a good idea to migrate to it. Only two things prevent me from doing this so far: it's not portable to previous version of Ruby, 1.8 (though it slowly becomes obsolete), and I find the old style look much more cute :-)

***

This was yet another typo that makes our programs behave differently than we expect them to. And again, it was just one character that breaks the expected behavior of if statements that implicitly convert the condition to boolean type. Unfortunately, while the new Ruby named parameter syntactic sugar could help, it sometimes looks even worse to me.

I hope this post will help you avoid the similar mistake if you code in Ruby.

I would like to end with a joke about a mythical C++ programmer leaving hist last commit after having been fired:

Recently I ran a software update from Ruby on Rails version 2.3.5 to 2.3.14. You might have noticed the "maintenance" message all pages were redirected to last Sunday. Of course, after the update completed, I clicked on a bookmark entry "My Blog" in the browser window, and, having acknowledged the correct loading, prematurely considered the update completed well.

I was wrong this time. But first,...

How I perform updates on my webserver

To automate the update process, I use a couple of nonstandard scripts. During the update, the web server becomes nearly unusable, so, instead of providing poor experience to users, I decided to show a "maintenance" page while the update is in progress.

In my virtual host Apache configuration file, I use a special variable, COLDATTIC_MAINTENANCE, to distinguish whether it should ask Mongrel for a web page, or should show a simple maintenance.html page. Here's how virtual host config looks like:

Now, the apache is serving maintenance pages. I edit the config /etc/conf.d/apache2 back, and remove the maintenance define. However, I do not restart the server with new settings!

I run a system update script, which, essentially, looks like update-software ; /etc/init.d/apache2 restart, so the server is automatically restarted without maintenance mode after the update is completed.

Go to movies, since the update usually happens on early East Coast Sunday mornings, when it's Sunday Night in Russia, a good time to relax;

Come home, log in to server, and deal with update failures >_<

I do not do the updates quite often, and they go well. After the update last Sunday, I noticed a sudden decrease of visits. This could be just a random spike, but after the visit number had decreased to less than ten the next day, I realized that the update broke something. I entered coldattic.info into the browser. It should have redirected to http://coldattic.info/shvedsky/pro/blogs/a-foo-walks-into-a-bar/, but what I saw was a raw HTML text of the blog page!

How Ruby went off the Rails.

How come? I checked the logs of the website, and notices an abnormal stacktrace:

It only happened when the page request should result in redirect, but it worked for direct page accesses (that's why, perhaps, the visit counter was greater than zero: people could just user their bookmarks).

I ensured the localhost version is synchronized with the production, and launched WeBrick via ./script/server to play with the website outside the production server environment. But on my local machine it worked well! (No wonder, because there's no entry of my web application in the stack trace; the whole trace consists of web server and framework subroutines.)

Solution: monkey-patching

I googled for the error, and it appeared to be more than half a year old. This bug entry is, I guess, the first place where the error was spotted. In a nutshell, new version of Rails seems to have employed a previously unsupported use-case of Rails' web server interface, Rack. It interfered with, supposedly, partly incomplete implementation of Mongrel, a specialized HTTP webserver for Rails. And mongrel updates still haven't made its way to Gentoo distribution my web server runs (-1 to Gentoo maintainers reputation counter!).

The solution a lot of people (including me) used is here. It is a small Ruby file that should be placed... into your application startup directory (for Rails it's config/initializers/)! You don't have to patch your system libraries and make new packages for your distributions: all you have to do is to add a file to the userspace. The code there uses a well-known feature of Ruby, monkey patching. It alters the implementation of external library classes. I already demonstrated how this could be used to add some Perl-like features to Ruby.

So, fixing others' bugs is another, though less legitimate, way to employ monkey-patching.

***

Why did this happen in the first place? Two reasons:

Insufficient testing after software updates. I should have tried to enter the site as an anonymous user, instead of clicking on my own bookmarks. Without the fix above, the site worked well if the user who accessed it had cookies!

Deploy the exactly same environment on testing machine as on the production, and perform the tests there before upgrading.

I thought that Mongrel and the whole Rails stack was stable, but, I guess, it suffers from fragmentation. Ruby on Rails stack consists of a lot of components. While this fosters innovations, the interaction between their developers is not tight, and there also is no common governance; this are the reasons why such strange bugs get shipped in stable versions.

Sometimes, when you read too many language standards and various specifications, you start using these weird words. Undefined Behavior is the ultimate evil, a raptor that storms in when you dereference NULL pointer, use freed memory, or violate any other preconditions, which are placed in every dark corner just to keep you awake. Futile attempts to make jokes aside, it's the wording that shows that compiler, library or system can guarantee nothing if you do that.

In most cases, nothing serious happens. Under the most common violations, such as dereferencing memory pointed to by an invalid pointer, you just get "Segmentation fault" (or "Access Violation" if you use an alternative OS) fatal error.

The Raptor Exists

A couple of days ago, I was trying to get wi-fi working at work and downloaded drivers from vendor's website. They happened to belong to the other kernel version, but I installed them, hoping that Undefined Behavior won't bite me much. When I rebooted, after the attempt to do network startup, something undefined broke there. And I observed the following picture...

The raptor was right here, already chewing my hand and smiling with an anime smiley ^_^.

So, guys, if you still don't believe in these tales about raptors that crush you if you do something unwanted, you'd better start believing. You have been warned.

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